Studies designed to elucidate the mechanism of action of cyclic AMP in the production of smooth muscle relaxation are in progress. We are attempting to purify and characterize various substrates (soluble, contractile, and membrane) for the cyclic AMP dependent protein found in tracheal smooth muscle. Anticpated future studies are designed to investigate the physiological importance of this enzyme and its substrates, specifically to link phosphorylation in vivo of these substrates to the actions of the beta-sympathomimetic drugs, as well as to correlate the degree of phosphorylation in vivo with the tone of the smooth muscle. BIBLIOGRAPHIC REFERENCES: Sands, J. Evidence against phosphorylation of cyclic AP dependent protein kinase from bovine tracheobronchial smooth muscle. Proc. Soc. Exptl. Biol. Med. 150:57-60 (1975). Sands, H. Actomyosin isolated from bovine tracheal smooth muscle. Proc. Soc. Exptl. Biol. Med. 150:299-302 (1975).